The present invention relates to active filters and more specifically to high gain, narrowband amplifiers
Communications systems transmit or receive signals in a portion of the radio frequency spectrum known as a frequency band. Usually, communications systems are forced to operate in close proximity to other bands. From the point of view of the communications system, all other signals can be regarded as interfering signals. Hence, communications systems attempt to use the band of interest while excluding interference from all other bands.
Therefore, most communications systems usually use some form of filter to allow the use of the information in the desired frequency band while excluding all other interference. These filters are often called bandpass filters or band limiting filters, and they may be used in receivers, transmitters, or both. Indeed, band limiting filters may be useful in any electronic system which requires frequency selectivity.
There are a number of desirable characteristics for band limiting filters. Firstly, it is often desirable for band limiting filters to be narrowband, that is, they are required to minimize the undesired information outside the band of interest that is passed by the filter. Sometimes it is also desirable for band limiting filters to provide gain thereby increasing the signal level, since often the communications system either transmits from a lower power signal source or receives a low power signal.
One known implementation of band limiting filters uses all passive components. Filters of this implementation are often known as passive filters. In general, passive filters are known to be implemented with combinations of lumped elements such as inductors and capacitors, or with equivalent distributed resonant structures such as ceramics, crystals, resonant transmission lines, and the like, or with hybrid lumped element structures with partially resonant structures, such as shortened transmission lines with capacitors or inductors added.
A limitation of the passive approach is that passive filters do not provide gain because there is no active element. Furthermore, passive elements will have manufacturing tolerances which, at radio frequencies, can be substantial enough to degrade the frequency performance of the filter. This degradation in frequency performance could result in a filter that is not narrowband enough or may be at the wrong frequency. Additionally, passive structures on a semiconductor substrate in an integrated circuit can be relatively large which may directly affect the cost of the system.
To achieve a signal gain as well as a narrowband response, an active filter is desirable. An active filter contains at least one active element, such as a transistor or diode, to provide a gain. Active filters may be realized as amplifiers where the frequency selectivity is achieved by the quality of the input and output impedance matching. Therefore, the amplifier may provide gain to a narrowband of the frequency spectrum while attenuating other out of band interferers.
A known type of active filter is called a super regenerative receiver. This device alternates itself from completely turned off, through an amplifying state, into an oscillating state. As the device passes through the amplifying state and close to the oscillating state, the amplifying bandwidth narrows. Thus, there is a period of time, after startup, but before the gain of the device saturates and oscillation begins, where the device is capable of narrowband amplification. This operating period, or state, is desirable for band limiting filter operation.
Traditionally, there have been a number of limitations to the super regenerative receiver approach. Firstly, it has been difficult to control the operation of the device during the desirable amplification stage. Specifically, the devices have not been sufficiently controlled so that the desirable amplifier state is maintained close to oscillation. Thus, the devices were usually pulsed on and off. This pulsed mode of operation limits the selectivity of the device due to sampling alias responses, and thus makes the device vulnerable to interference. Secondly, although in the amplifying state the devices can achieve very high sensitivity, they are difficult to tune and are highly non-linear. As a result of these factors, super regenerative receivers have seen limited usefulness in general high performance communications applications.
For the foregoing reasons, there is a need to provide an amplifier for high gain, narrowband signal amplification.
The present invention is directed to an amplifier for high gain, narrowband signal amplification.
An embodiment of the invention is an amplifier including a first circuit capable of oscillating and a second circuit for controlling the operating state of the first circuit between oscillation and close to oscillation. By operating close to oscillation high gain, narrowband signal amplification occurs. By operating between oscillation and close to oscillation, rather than between startup and close to oscillation, the amplifier is always narrowband.
According to an aspect of the present invention, there is provided an amplifier which includes a first circuit having a variable transconductance part and a frequency control part, and being capable of oscillating to generate an output signal in response to an input signal from an antenna and a controller for controlling the operating state of the first circuit between oscillation and close to oscillation so that high gain, narrowband signal amplification occurs. The controller has a circuit for generating an error signal through a comparison of the output signal of the first circuit and a Q reference level, a first filter for filtering the error signal to hold the first circuit at a desired Q, a circuit for generating a quenching signal of a sample rate for a quenched operation of the first circuit, a circuit for generating a resulting signal through a multiplication of the filtered error signal and the quenching signal and applying the resulting signal to the transconductance part to control transconductance, a circuit for generating a driving signal through a comparison of the output signal of the first circuit and a reference frequency, and a second filter for filtering the driving signal and applying the filtered driving signal to the frequency control part. A filtering bandwidth is narrow relative to the signal bandwidth if the filtering is active during a matched Q portion with the quench cycle.
Preferably, the amplifier further includes a circuit for detecting DC offsets on the output signal of the first circuit and feeding back a correction signal to the output signal of the first circuit to eliminate the DC offsets, and a circuit for detecting data from an output of the DC offsets detecting circuit. The output signal of the first circuit is Manchester encoded, and the feeding back is carried out in a limited bandwidth to less than that of a lowest frequency component of the Manchester encoded data.
Preferably, the amplifier further includes a baseband Q control circuit for inputting the output signal of the first circuit and applying an output signal thereof to the error signal generating circuit to control the Q of the first circuit by using background thermal noise in the baseband filter when the Q of the first circuit is sufficiently high.
Preferably, the error signal generating circuit includes a unit for autocorrelating the output signal of the first circuit, a unit for bandlimiting an output signal of the autocorrelating circuit to restrict the output signal of the autocorrelating circuit to baseband frequency components of the autocorrelation, a unit for setting the Q reference level, and a first comparing unit for comparing the bandlimited output signal and the Q reference level.
Preferably, the error signal generating circuit further includes a charging pump connected between the comparing circuit and the first filter.
Preferably, the first filter is a Q loop filter having an integrator and a differentiator to allow high gain at DC and early loop response to changes in state.
Preferably, the driving signal generating circuit includes a unit for limiting the output signal of the first circuit to removes excessive AM on the output signal of the first circuit, a unit for dividing an output signal of the limiting unit, a unit for controlling the division ratio of an output of the dividing unit so that a divided down signal of the output of the dividing unit equals the reference signal, a unit for generating the reference signal, and a second comparing unit for comparing the output of the dividing unit and the reference signal and generating the driving signal. The AM is reduced to allow the dividing unit to operate continuously.
Preferably, the driving signal generating circuit further includes a unit for dividing the reference signal. The reference signal is divided before the comparison of the output of the dividing unit to the reference signal.
Preferably, the driving signal generating circuit further includes a unit for gating the dividing unit and the reference signal dividing unit in a fashion that, in the off state, the gating unit retains the signals of the dividing unit and the reference signal dividing unit in a frozen fashion until the next gated on-state, to thereby prevent a frequency of the second filter from being jammed or interfered with by large unwanted signals.
Preferably, the driving signal generating circuit further includes a charging pump for converting an output of the second comparing unit into the driving signal to drive the second filter.
Preferably, the limiting unit is a limiter amplifier.
Preferably, the dividing unit is a prescalar circuit.
Preferably, the reference frequency generating unit is a crystal oscillator.
Preferably, the second comparing unit is a phase frequency detector.
Preferably, the second filter has an integrator and a differentiator.
Preferably, the DC offsets detecting circuit includes a low pass filter for filtering the autocorrelated output signal of the first circuit, and a DC reject loop for receiving and feeding back the correction signal to the filtered autocorrelated output signal, to thereby ignore signals present as large oscillating signals.